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The candela (symbol: cd) is the SI base unit of luminous intensity (perceived power emitted by a light source in a particular direction).


Since the 16th General Conference on Weights and Measures in 1979, the candela has been defined as follows:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.


The candela was based on an older unit, the candlepower, which was referenced to the luminous intensity of a "standard candle" of known composition.


The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometers. The human eye is most sensitive to this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. (See luminosity function).

A common candle emits about 1 cd. A 100 W lightbulb emits about 120 cd.

Although the definition of candela is now based on the watt, which is a derived SI unit of power, the candela remains a base unit of the SI system [1].

Historically, the candela was defined in terms of the black-body radiation emitted by 1/60 of 1 cm2 of platinum at its melting point. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition.

SI photometric light unitsEdit

Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lm•s units are sometimes called Talbots
Luminous flux F lumen (= cd•sr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd
Luminance Lv candela / square metre cd/m2 also called luminosity
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy lumens / watt lm/W ratio of luminous flux to radiant flux, maximum possible is 683


External linksEdit

Candela: The SI Unit of Luminous Intensity

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